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This reverts commit b43fa6db.

Reason for revert:
Caused three bugs open for flaky tests:
https://bugs.chromium.org/p/chromium/issues/detail?id=869043
https://bugs.chromium.org/p/chromium/issues/detail?id=868769
https://bugs.chromium.org/p/chromium/issues/detail?id=868839

The speculation is that new calls to ScanDirForExternalWebApps() are not being done on the correct thread.

Original change's description:
> Allow (auto-installed) external web apps
> 
> These are the Web App analogs of external extensions, described at
> https://developer.chrome.com/apps/external_extensions
> 
> On start up, *.json files (often but not necessarily named
> external_extensions.json) are scanned in a number of directories. Prior
> to this CL, such .json files can install regular extensions (.crx files)
> or, on Chrome OS, Android apps. New in this CL is being able to install
> Web Apps (sometimes known as Progressive Web Apps or PWAs).
> 
> For example, the chrome::DIR_USER_EXTERNAL_EXTENSIONS path (defined in
> chrome/common/chrome_paths.h) can correspond to file system directory
> like "$HOME/.config/chromium/test-user/.config/chromium/External
> Extensions". Placing a foo_bar.json file containing:
> 
> {
>   "web_app_manifest_url": "https://www.chromestatus.com/static/manifest.json",
>   "web_app_start_url": "https://www.chromestatus.com/features"
> }
> 
> in that directory will install the Chrome Platform Status web app.
> 
> Bug: 855281
> Change-Id: I716fead81d407076a0dda32f5c0b3e8869351c83
> Reviewed-on: https://chromium-review.googlesource.com/1127214
> Commit-Queue: Nigel Tao <nigeltao@chromium.org>
> Reviewed-by: Dominick Ng <dominickn@chromium.org>
> Reviewed-by: Alexey Baskakov <loyso@chromium.org>
> Reviewed-by: Giovanni Ortuño Urquidi <ortuno@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#578944}

TBR=ortuno@chromium.org,loyso@chromium.org,dominickn@chromium.org,nigeltao@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 855281
Change-Id: I976928b151910bcc39ea372a6f80d9d833fc8253
Reviewed-on: https://chromium-review.googlesource.com/1155496Reviewed-by: Roger Tawa <rogerta@chromium.org>
Commit-Queue: Roger Tawa <rogerta@chromium.org>
Commit-Queue: Peter Boström <pbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579247}